A good deal of recent literature dealing with optical signal processing techniques concerns the formation of an electromagnetic radiation pattern for phased array antennas. Except for the approaches which utilize switched fiber architectures, most beamforming systems, however novel, are limited to narrowband operation. Aside from the switched fiber systems, other approaches typically use an optical heterodyne process to produce an RF phase shift which restricts its use to narrowband operation for phased array applications. To achieve wideband performance requires variable time delay to steer the transmitted/received RF beam. Though something as simple as a length of optical fiber can act as a delay line, the ability to attain variable delay has required the use of complicated and lossy switching architectures.
A system which results in many variable delay lines can be implemented using a single optically tapped acousto-optic (A/O) modulator; Toughlian et al., "A photonic variable rf delay line for phased array antennas" IEEE J. Lightwave Technology 8(12), 1824-1828 (1990). This approach only functions efficiently in the transmit mode of operation, where one modulating signal is to be delayed and distributed to individual detectors, one for each antenna element. In the receive mode of operation, the incoming signal from each antenna element must first be delayed and then summed to a single detector. Thus, in the receive mode of operation, the non-reciprocal character of acousto-optic modulators requires the use multi-channel A/O cells i.e. one A/O cell per antenna element. This makes their use infeasible in any application where power and weight are the primary concerns.
A phased array receive system is similar to that of a transversal filter system. A basic transversal filter (parallel) configuration consists of an RF input which is split and sent to different delay lines possibly with amplitude weighting. These weighted delayed versions of the input signal are summed to yield the desired response. The general concept is illustrated in FIG. 1a showing an array of variable delay lines 1, input terminal 5 and summer 3 coupled to the outputs of the delay lines. FIG. 1b shows the analogy to the phased array antenna case. There we see a point source radiator 5' in the far field which propagates to the antenna array where each antenna element receives a delayed version of the source. The goal is to choose values of delay so as to make the antenna array "listen" in a preferred direction. By varying the delays the antenna array can be made to listen in different directions. Consequently the phased array receive system is one specific application of transversal filter signal processing.
It is desirable to provide a new optical processor for the control of a broadband phased array receive system or any other application where an adaptive transversal filter is required.